Battles over what's true and what's false come at us today with such furious regularity that it's hard to make sense of it all with the sober detachment and systematic analysis that's needed. Yet for all the blur and noise, it seems safe to say that a Rubicon was crossed this year in the skirmish over whether President Trump, in talking about immigration with legislators, used a particular eight-letter scatological epithet.

In that controversy - which already seems a lifetime ago - Trump supporters who'd been in the room with the president didn't deny, at first, the reports that he had used this nasty vulgarism to describe Haiti and Africa. They seemed to hope the matter would just go away. After the president let loose with his taunts of "fake news," however, a funny thing happened: Those once-reticent allies suddenly recalled with sterling clarity that Trump had absolutely said no such thing.

This, then, wasn't just Trump trying to rewrite reality for short-term personal or political gain. He was joined by political confederates cynically abetting his gaslighting. It's bad enough for democracy when millions of Americans believe one powerful man's brazenly false assertions. But if the president can count on a legion of yes-men to echo his falsehoods and give them a sheen of independent verification, then the task of rediscovering common ground in our political debates will grow even harder than it has already become.

Given Trump's practice of sowing confusion, it may seem perverse to publish a book arguing that most of what we assert to be true - in politics and in other realms of experience - is in fact just partial and selective truths, shaped consciously or unconsciously by our attachments, beliefs, wishes and preferences. But the merit of Hector Macdonald's "Truth: How the Many Sides to Every Story Shape Our Reality" is precisely that it forces us to remember how few claims can be definitively established as beyond dispute. Although politics today is, unquestionably, rife with lies and falsehoods, our most intractable conflicts occur when two (or more) sides can each defend the truthfulness of their facts but can't agree on which facts deserve priority or what their implications are.

Our arguments about what's true often mask deeper disagreements about values or worldviews.

This position shouldn't be mistaken for relativism. Rather, Macdonald's claim derives from an insight put forward most famously by the philosophers of the early-20th-century Vienna Circle: the notion that apart from the a priori truths found in mathematics and logic, and the kind of empirical facts that can be scientifically verified, few statements can be pinned down as unmistakably true. Notions about religion, morality, politics and any number of other subjects may enjoy wide or even universal endorsement, and may have some evidence to support them, but they aren't technically true in the way that "2 (plus sign) 2 (equals sign) 4" is, or even in the way that "H2O (equals) water" is.

In the hands of a philosopher or trained political theorist like Michael Sandel, Harry Frankfurt or Kwame Anthony Appiah, a book-length exploration of the different ways we construct what we call truth could be an enlightening exercise.

Regrettably, "Truth" is written in the self-consciously superficial style of the airport business book, seemingly in the hopes of attracting readers of best-selling authors such as Malcolm Gladwell and Cass Sunstein, who breezily deploy anecdotes from a smattering of situations to illustrate clever phenomena.

A self-described "expert in business storytelling," Macdonald says he is writing for people who consider themselves "communicators" - a term most of us who speak and write and express ideas for a living would disavow as silly jargon. Macdonald's end-of-chapter bullet points don't help.

But while the book's style grates, and while its arguments are only desultorily pursued, its elaboration of the way "truths collide," as Macdonald puts it, has a kind of handy value in what many people are describing as our "post-truth" era. That label has always been imprecise, because it isn't really truth that's disappearing: Two plus two still equals four, even if the Ministry of Information tells us otherwise. What's vanishing are the sources of authority who enjoy credibility from people with divergent underlying values or viewpoints.

Whatever we call it, though, a year into Trump's presidency it's clear that we're undergoing a shift in how truth is understood in our politics.

To be clear, this phenomenon didn't start with Trump. Indeed, what makes Trump's assault on the truth so dangerous is that well before his presidency - even before his odds-beating half-court-jumper of a presidential candidacy - we were already deep into a partisan struggle over truth.

It was under George W. Bush, after all, that Stephen Colbert coined the term "truthiness"; it was under Bush that liberal journalists like David Corn and Joe Conason wrote books with titles such as "The Lies of George W. Bush: Mastering the Politics of Deception" and "Big Lies: The Right-Wing Propaganda Machine and How It Distorts the Truth" - not to mention Al Franken's "Lies and the Lying Liars Who Tell Them: A Fair and Balanced Look at the Right."

One can go further back, to the 1990s, to see the confessional ethos of daytime television infiltrating the new forms of online journalism, leading to personal anecdotes replacing reported experience and unearned righteousness superseding patient argumentation.

That was the era that first popularized the grating phrase "speak my truth," which ironically undermines its own demands to be heard with its implicit relativism.

For the sake of convenience, though, the new era might be dated to the publication of a 2004 New York Times Magazine article by journalist Ron Suskind about Bush's "faith-based" presidency.

In the piece, a top White House aide mocked Suskind for being part of "what we call the reality-based community" - people who "believe that solutions emerge from your judicious study of discernible reality." And the aide went on to declare, in a remark often quoted since: "That's not the way the world really works anymore. We're an empire now, and when we act, we create our own reality."

The Bush White House certainly wasn't the first to play fast and loose with the facts.

But it was under Bush that the White House finally realized a decades-long conservative project begun under Richard Nixon: to set up an alternative system of think tanks, experts, media outlets and other sources of authority that could counter the ostensibly objective claims of nonpartisan experts whose judgments tended to buttress liberal policy preferences. T

hanks to the creation of this counter-establishment, the Bush White House could find economists who said its tax cuts would not worsen the deficit, scientists who saw God's hand in evolution, intelligence analysts who said Saddam Hussein had sought to reconstitute his nuclear program and NASA employees who insisted that the Big Bang was just a "theory."

In reaction to the spread of pseudo-expertise - and to Bush's proud reliance on his gut and intuition to make decisions - many liberals and administration critics proudly declared themselves "reality-based" and tried to enlist scientific and scholarly evidence in support of their arguments.

This determination to hold fast to truth, not to let those in power get away with large-scale lying, is today more essential than ever to sustaining the health of our democracy. At the same time, however, denizens of the reality-based community have sometimes lost sight of the difference between fact and opinion. Or they have been reluctant to concede that even expert opinion can be open to question or rival interpretations. Here's where Macdonald's unfashionable conclusions warrant attention.

Polarization, of course, has made it harder than ever to establish agreed-upon truths. There's no easy way out. Impeaching Trump, or electing a new president in 2020 or 2024, won't suddenly lift the spell.

Our retreat into separate silos, where we consume our own news sources and associate with our own groups that share our own values and opinions, has led us all to view the world in starkly different ways from our political antagonists - and it appears also to have made us more antagonistic. The only thing we seem to be able to agree on is that our ways of seeing the world are irreconcilable - an observation so commonly made as to be a cliche. Or is it a truism?

David Greenberg is a professor of history and journalism and media studies at Rutgers University and the author, most recently, of "Republic of Spin: An Inside History of the American Presidency." He wrote this piece for The Washington Post, where it first appeared.